The present invention relates to fluid monitoring and fluid analysis. As used herein, a fluid can be either a liquid or a gas. The invention optically monitors fluids on-line (i.e., while in use) such as, e.g., lubricants, natural and/or synthetic motor oils, standard additives and/or adjuncts, combustion engine fuels, other hydrocarbon-based fluids used in transportation and industrial applications. The present invention further relates to an apparatus for on-line analysis of a fluid's quality and/or condition using a fluid's optical absorption to determine, e.g. base fluid, amount or depletion of a performance additives, contamination with unwanted liquids or solids, general degradation of the fluid due to chemical breakdown, or other variables associated with a fluid's type, condition or quality.
Fluids are a critical component for proper operation of many devices and/or processes. For example, lubricants are needed for an internal combustion engine to efficiently provide power over long service life; high quality fuel is needed for proper engine operation with minimal emissions; and metal working fluid is needed for rapid waste metal removal and maximum tool life. Optimum performance is achieved when the fluid in question is of a proper quality for the application. For a particular application, a fluid preferably includes an appropriate base fluid and proper performance additives, e.g., corrosion inhibitors, friction modifiers, dispersants, surfactants, detergents, and the like. During use or consumption, a fluid's condition should remain within determined limits, i.e., chemical and/or other fluid changes should be within proper performance specifications.
Often, device owners and/or process operators depend on suppliers to provide proper quality fluids, and depend on regular level checks and fluid replacement to maintain proper fluid condition. However, the foregoing is inherently limited and does not provide protection against accidental fluid substitution, or catastrophic fluid failure. In addition, regularly timed maintenance intervals can be wasteful if a fluid, with remaining useful life, is prematurely replaced or refreshed. Such premature maintenance, however, is often desirable rather than risk damage or inefficient operation due to overly degraded fluids. In any event, owners and/or operators can minimize fluid maintenance costs without risking damage or inefficient operation if fluid maintenance occurs only at or near the end (natural or otherwise) of fluid's usefulness. Hence, an on-line fluid monitoring method and apparatus is desirable to provide substantially “real-time” determination of a fluid's initial quality and of a fluid's continuing condition during use.
Infrared (IR) spectroscopy is a tool that has long been used to obtain information about a fluid's quality and condition. IR spectroscopy apparatus, typically, either transmits broad-frequency IR light through a fluid of interest, or reflects broad-frequency IR light from a surface of an IR-transparent element of sufficiently high index-of-refraction (known in the art as an internal-reflectance-element or IRE) in contact with the fluid of interest. If no fluid is present in the apparatus, all light frequencies (wavelengths) are equally transmitted or reflected. When a fluid is present, however, IR light is absorbed at specific frequencies associated with particular fluid chemical groups. The amount of IR light absorbed is a function of the particular chemical concentration. Some fluid chemical groups also absorb IR light over a wide frequency range, for example soot in a heavy-duty diesel engine lubricant. Concentration of these groups is determined by comparing shift in IR absorbance at one or more frequencies where absorption due to other groups should not occur; i.e. the shift in baseline absorption is determined. Hence, by analyzing IR spectra for valleys and change baseline absorption, information of fluid quality and condition can be determined.
Despite proven analytical capabilities, IR spectroscopy has remained primarily a laboratory tool. In particular, on-line use of IR spectroscopy for real-time fluid analysis has been essentially limited to monitoring chemical processes in stationary plants due to IR apparatus cost, size, shock and vibration sensitivity, and complexity of data analysis issues. Most of these issues are directly related to monitoring the entire IR spectrum for optimum fluid analysis, which requires the IR apparatus to have relatively large and expensive movable components that are sensitive to shock and vibration, and also requires the apparatus to include relatively expensive data analysis hardware and software.
There are, however, on-line applications where adequate fluid analysis can be obtained by monitoring only a number of discrete frequencies instead of the entire IR spectrum. As used herein, a monitored frequency is a range of frequencies that has a defined central-frequency and bandwidth; hence, all light paths that are described herein for use in monitoring a frequency includes sources, detectors or filters that emit, detect or filter a range of frequencies that has a defined central-frequency and bandwidth. Typically, the number of discrete frequencies needed to adequately analyze a fluid is less than 20, and more typically less than 5; however, applications can require greater than 20 frequencies for adequate analysis. These discrete-frequency applications minimize required data analysis hardware and software, and several approaches have been taken to address the other issues of on-line IR fluid analysis described above.
One discrete-frequency approach uses a fixed prism and discrete detectors at fixed location relative to the prism to monitor the desired frequencies. While this approach minimizes shock and vibration sensitivity issues, apparatus size remains relatively large due to the optical path length needed to get sufficient frequency resolution, that is, to minimize the bandwidth of the frequencies received by the detector. If the apparatus is required by the application to operate over a wide temperature range, operational stability can be an issue due to thermal expansion affecting the optical path through the prism. Also while this approach has reduced apparatus cost, the precision needed to manufacture the prism and placing the detectors still has significant cost.
Another discrete-frequency approach uses a filter, a source or a detector of a desired IR frequency to create a light path, which can include an IR transparent IRE in contact with a fluid, that is cost effective, compact, robust, and minimally affected by temperature fluctuations. Current designs, however, have not effectively solved the problems of how to monitor more than one IR frequency in a single sensor package, especially when a different number of internal reflections is need for each frequency to properly resolve changes in a monitored fluid.
The present invention overcomes limitations of previous approaches to discrete frequency IR apparatus. The present invention is a compact, low-cost, and robust IR apparatus with an IRE that can monitor a multitude of IR frequencies, each having a determined number of reflections in the IRE, over a wide temperature range. The apparatus can be used on-line to provide information relevant to the real-time quality and/or condition of a fluid.